1. Field of the Invention
This invention relates to glasses having good stability to radiation useful as cladding glasses in space and terrestrial applications, and to cladding panes composed of such glasses.
On irradiation with high energy radiation typically encountered in space, glass tends to discolour, reducing the transmission of the glass and increasing its solar absorptance. Thus, radiation stability is a particular requirement of glasses used as cladding glasses in space applications, for example, as solar cell cover slips or as the glass substrates of second surface mirrors used as cladding to protect spacecraft from overheating.
2. Description of the Prior Art
It is known, for example from EP 0 261 885A1 and EP 0 505 061A2, to use borosilicate glasses, stabilised against the effects of irradiation by the incorporation of cerium (typically in amounts of 2% to 5% by weight), for production of solar cell cover slips having a high transmission in the visible and infra-red regions of the spectrum. Cerium has very broad absorption bands in the ultra-violet region of the spectrum at 240 nm and 315 nm. This absorption in the ultra-violet may be beneficial when the glasses are to be used in solar cell cover slips, for example, in protecting the adhesive used to bond the cover slips to the cells from ultra-violet radiation which would otherwise tend to degrade the adhesive. However, when the same base glasses are used, with a reflective coating on the back surface, as second surface mirrors to clad the exterior surface of a space craft and reflect unwanted solar radiation incident upon it, the absorption in the ultra-violet leads to an undesirable build-up of heat in the glass.
Thus there is a need for a method of stabilising a high transmission borosilicate glass to radiation especially radiation encountered in space, which does not rely on the use of cerium (or any other element which absorbs significantly in the spectral region from 250 nm wavelength to 2500 nm wavelength). It has now been found, and the discovery forms the basis of the present invention, that borosilicate glasses may be stabilised against radiation by inclusion of barium.